This invention relates to a method of toughening a glass sheet by heating the glass sheet to a temperature above the strain point of the glass and cooling the heated glass sheet while the glass sheet is transferred horizontally in a lying state, the method being particularly suitable for application to a relatively thin glass sheet, viz. glass sheet thinner than about 4 mm, for use in an automobile side or rear window.
In the recent automobiles there is a trend toward enlargement of the window areas while reducing the thickness of the window glasses. Accordingly it has become more and more important to efficiently and reliably toughen relatively thin and relatively wide glass sheets.
There are official regulations specifying the manner of fracture of toughened glass sheets for use as side and rear windows of automobiles. For example, both the British standard and the E.E.C. standard specify the minimum and maximum numbers of glass particles in any 5 cm.times.5 cm square traced on the fractured glass sheet further requires that the fractured glass sheet should not contain elongated particles longer than 6 cm (such elongated particles are called "splines").
However, it is not easy to toughen glass sheets thinner than about 4 mm by a conventional air quenching method so as to fully meet the official regulations, because in the case of a thin glass sheet it is difficult to create and maintain a sufficient gradient of temperature in the thickness of the glass sheet during the quenching process. To overcome the difficulty it is prevailing to differentially quench a glass sheet so as to distribute in the glass sheet relatively highly toughened regions and lesser toughened regions in a suitable pattern. In practice, to enhance productivity it is often that the quenching is performed while glass sheets in a horizontally lying state are successively transferred through a furnace and a cooling station. In such a case the difficulty of desirably toughening a thin glass sheet augments because the distribution of differentially toughened regions in the glass sheet is affected by the unidirectional movement of the glass sheet.
For example, U.S. Pat. Nos. 4,178,414 and 4,182,619 relate to a method of toughening a relatively thin glass sheet while it is advanced horizontally through a quenching station by directing cool air jets to each face of the heated glass sheet from a lattice-like array of nozzles and show to pulse the air jets at a repetition frequency related to the speed of advance of the glass sheet and also to slightly incline rows of the nozzles to the direction of advance of the glass sheet. U.S. Pat. No. 4,198,463 relates to an analogous method and shows to toughen a relatively thin glass sheet of a trapezoidal shape so as to relatively highly toughen a strip-shaped region near the longer of the parallel sides of the glass sheet. JP No. 1-38058 also relates to an analogous method and shows to toughen a relatively thin glass sheet so as to relatively highly toughen a plurality of stripe-like regions which are elongate in the direction of advance of the glass sheet.
The methods of the above patents will have some advantages, respectively. However, in my view any of the hitherto proposed quenching methods is not yet fully satisfactory for desirably quenching a relatively thin glass sheet while it is advancing horizontally, and particularly for reliably preventing the appearance of splines in the fractured glass sheets.